Technical Field
The present invention relates to an insulating material, a capacitor, a circuit board, and a method of manufacturing an insulating sheet.
Related Art
In recent years, efforts have been made to use a fiber assembly, configured by using cellulose, as an insulating material.
Cellulose is a biodegradable, and environment-conscious material, and is moreover expected to contribute to weight reduction of electronic parts or electronic devices, or, weight reduction and flexibilization of electronic parts or electronic devices.
For example, JP-A-2008-242154 discloses an invention regarding a flexible board which is configured by a composite material sheet obtained by compounding a cellulose nanofiber assembly with an amorphous rigid resin, and is directly laminated with a sheet glass (also referred to as “prior art 1”, hereinafter).
The prior art 1 is aimed at improving the flexibility of the board by thinning the glass, and at reinforcing the flexible board through lamination of the composite material sheet.
On the other hand, JP-A-2008-242154 gives no description on a board mainly composed of cellulose nanofiber.
The insulating material such as insulating sheet, if successfully configured by mainly using a fiber assembly made of cellulose, would be preferable since lightness in weight and flexibility, inherent to cellulose, will be fully exhibited also in the insulating material. To configure the insulating material by mainly using cellulose has, however, faced technical problems.
More specifically, dielectric constant (∈) exhibited by cellulose per se is known to be 8 or around.
Efforts of paper-making, aimed to obtain the cellulose material such as pulp in a sheet form suited to general use for electronic parts, have however encountered the problems below. The dielectric constant (∈) of paper made by general method of paper-making is measured to be 2.0 to 2.5. That is, the dielectric constant (∈) of paper made into a sheet form is strikingly smaller than that of cellulose. The present inventors presumed that such reduction in dielectric constant is ascribable to avoid (air layer) which is formed among cellulose fibers in the process of paper-making.
The present inventors finely disentangling cellulose into nanofiber by a known means, and formed a cellulose nanofiber sheet. It was confirmed from our measurement that the cellulose nanofiber sheet showed the dielectric constant (∈) improved up to 5 or around. It is, however, necessary to further elevate the dielectric constant, in order to use the fiber assembly composed of cellulose nanofiber as an insulating material for electronic parts.